Extract checklist logic into testable utility module:
- ChecklistItem type with parentId field for single-level nesting
- parseChecklist/serializeChecklist with 2-space prefix for children
- toggleItemWithCascade: parent check/uncheck cascades to children
- indentItem/outdentItem with sibling adoption rules
- 21 unit tests covering parse, serialize, toggle, indent, outdent
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>